The efficiency of existing marine waterjet propulsor designs are limited and optimum efficiency has to be tailored to a specific operating condition of the marine vehicle in which the propulsor is to be installed. The efficiency ofpropulsors vary with vehicle speed and decreases substantially at off-design conditions, particularly at lower craft speeds. Even while operating at optimum efficiency and the intended design conditions, existing waterjet designs suffer from areas of poor flow quality, uneven pressure distributions, flow circulation, flow separation and impeller cavitation. These undesirable attributes limit fluid mass flow and velocity, thereby reducing potential thrust for any given power input. Many of these effects are directly attributable to the multitude of negative influences arising from the positioning and geometry of the waterjet impeller shaft.
FIG. 8A is a prior art illustration of a typical waterjet propulsor 10. As shown, the propulsor 10 includes an impeller shaft 20 with impeller blades 30 and impeller hub 80. The shaft 20 has a substantially circular cross section, and is mounted at the forward end via a forward bearing arrangement 40 to the propulsor frame structure 50. The shaft is mounted at the aft end via an aft bearing arrangement 41 to the stator/nozzle assembly 70. FIG. 8A also shows the intake 60, through which the propulsor inlet flow is developed. As shown, the shaft 20 is situated entirely within the flow field, indicated by arrows F, created upstream of the impeller. The shaft 20 presents an obstruction to flow creating turbulence and a large wake region within the housing of the propulsor, downstream of the shaft. This introduces turbulence and extreme variations in the velocities and pressures of the flow field entering the plane of the impeller blades 30.
The turbulence introduced is due to the location of the shaft, the shape of the shaft, and the rotational movement of the shaft. As shown in FIG. 8A, the impeller shaft 20 is positioned directly in the path of the intake fluids, thereby obstructing the flow into the impeller blades 30. Being a right circular cylinder, the cross sectional shape of the impeller shaft 20 possesses one of the highest drag coefficients for a non-blunt object. FIG. 8B shows the character of the drag coefficient as a function of Reynolds number for fluid flow over objects of different cross-sectional shapes, including that of a circle and an ellipse. It should be noted that in actuality, because of the angle β (shown in FIG. 8A) at which the flow approaches the shaft 20, the shaft cross section over which flow occurs may be more akin to an ellipse, which is more efficient than a circle, but less efficient than an airfoil.
Additionally, as opposed to the simple case of flow over an immersed stationary cylinder or ellipse, the surface of shaft 20 is not stationary, but rotating. Consequently, additional detrimental boundary layer effects associated with the tangential velocity of the shaft surface are introduced due to the shaft rotation (Magnus effect). The prior art does not provide a propulsor structure that minimizes the deleterious effects of the shaft to optimize the operation of the propulsor.